This invention relates to a web material inspection system of the type that utilizes a rotating multifaceted mirror to direct a scanning beam of radiation towards a radiation collecting arrangement suitably disposed with respect to the web. In particular, the invention relates to an inspection system in which the signal representative of the web being inspected is compensated for deviations superimposed on that signal due to variations in the reflectivity among the various mirror facets and normalized for variations in the optical efficiency of the radiation collecting arrangement.
Opto-electrical web inspection systems using the calibrated flying spot scan technique to automatically inspect web material, such as webs of X-ray film or fabric, are known. Exemplary of such an apparatus is that disclosed in U.S. Pat. No. 3,843,890 issued to Anthony, Jr. et al. and assigned to the assignee of the present invention. The web inspection apparatus generally comprises a source of scanning radiation, means for generating a beam of radiation and for traversing the beam in a scan across the web, and a radiation collecting arrangement positioned with respect to the web and responsive to the radiation either reflected from or transmitted through the web for generating an electrical signal representing a predetermined physical property thereof. The radiation source is typically a laser. The beam of collimated light from the laser is directed toward the web by a multifaceted rotating mirror disposed within a scanning and focusing optical assembly. The radiation beam appears as a spot traversing the web in a predetermined scan direction. The radiation collecting arrangement, such as a tapered light-conducting rod, collects the radiation reflected by or transmitted through the web, as the case may be, and directs that radiation to a detector. The detector, such as a photomultiplier tube, generates an electrical signal representing a predetermined physical property of the portion of the product web scanned by the radiation beam.
The electrical signal representative of any one scan across the product web is typically represented by a voltage pedestal signal output from the photomultiplier tube. The voltage pedestal signal for a given scan i may be expressed mathematically as a function v.sub.i (x(t)) as set forth in Equation (1), EQU V.sub.i (x(t))=K.sub.i (x(t)).multidot.P.sub.i (x(t)) (1 )
where
x(t) is the distance traversed across the web measured from an initial position, x.sub.o, at the start of each scan across the product web as a function of time t,
P.sub.i (x(t)) is the electrical signal representative of a predetermined physical property of the portion of the web scanned by the radiation beam at time t during scan i. This term is also referred to as the product characteristic, and
K.sub.i (x(t)) is an attenuation function, valid for scan i, which is mathematically defined as EQU K.sub.i (x(t))=I.sub.o (t).multidot.R.sub.i.sbsb.F .multidot.G.sub.o (t).multidot.G.sub.e (t).multidot.E.sub.o (x) (2)
where
I.sub.o (t) is the radiation source intensity,
R.sub.i.sbsb.F is the reflectivity of a mirror facet on the i-th scan, where i.sub.F =(i MODULO F), i is the scan number
(i=0,1,2. . .) and F is the number of facets,
G.sub.o (t) is the optical system gain,
G.sub.e (t) is the electrical system gain, and
E.sub.o (x) is the optical efficiency of the radiation collecting system.
As seen from Equation (1) the voltage pedestal signal v.sub.i (x(t)) output from the photomultiplier tube is a function of both position (with respect to the web) and time. The product characteristic component P.sub.i (x(t)) of the voltage pedestal is that portion of the voltage pedestal due to reflectivity or transmissivity of the scanning radiation by the product web. The voltage pedestal signal vi(x(t)) is also functionally related to an attenuation function K.sub.i (x(t)) which is itself a function of position across the web and time.
Examples of causes for variations in the radiation source intensity I.sub.o (t) may include power supply voltage level changes and aging of the laser optics or laser components. Optical system gain variations G.sub.o (t) may originate as the result of photomultiplier tube aging, dust on the radiation collecting arrangement or on the entrance and exit ports of the optical scanning and focusing assembly. Electrical system gain variations G.sub.e (t) may derive from changes in electronic component characteristics in various of the electronic stages. Variations in these parameters generally occur in the same time scale and may be correctable utilizing automatic gain control circuitry. As a consequence the effect of these parameters on the voltage pedestal signal v.sub.i (x(t)) may be minimized or eliminated.
On the other hand, scanning mirror reflectivity variations R.sub.i.sbsb.F occur over a much shorter time since they may originate as a result of uneven reflective properties among various of the F facets of a rapidly rotating multifaceted mirror. Although the reflectivity across the span of any one facet is relatively constant, variations in reflectivity can occur from facet to facet. Haze and dust are generally not included as causes of mirror reflectivity variations because it is assumed that the buildup of haze and dust on each of the facets in the multifaceted rotating mirror is relatively uniform. The optical efficiency of the radiation collecting arrangement E.sub.o (x) is a function of the distance from the detector at which the reflected or transmitted radiation impinges upon the collecting rod and is invariant with time.
In order to make the voltage pedestal signal v.sub.i (x(t)) output from the photomultiplier tube dependent only upon the product characteristic P.sub.i (x(t)) of the web being inspected it is necessary to compensate in some manner for both the effects of variations in mirror reflectivity among the mirror facets and for the variations in the optical efficiency of the radiation collecting arrangement.